1. Field of the Invention
Apparatuses consistent with the present invention relate to a light guide plate and an illumination apparatus for a display device using the light guide plate, and more particularly, to a light guide plate having multi-periodic micro patterns, which can minimize chromatic dispersion, and an illumination apparatus for a display device using the light guide plate.
2. Description of the Related Art
In general, non-emissive displays such as liquid crystal displays (LCDs), typically require illumination apparatuses such as backlight units. FIG. 1 is a cross-sectional view of a conventional illumination apparatus for a display device, the illumination apparatus including a hologram-based light guide plate. Referring to FIG. 1, a conventional illumination apparatus 10 for a display device includes a light guide plate 12 having a top surface on which a micro pattern 13 is formed, a light source 11 disposed at a side of the light guide plate 12, and a light diffuser sheet 15 uniformly diffusing light emitted from the top surface of the light guide plate 12.
A white light emitting diode (LED) is generally used as a light source 11 of the illumination apparatus 10 for the display device. White light emitted from the light source 11 is obliquely incident on a first side surface of the light guide plate 12, which is formed of polymethyl methacrylate with high light transmittance, adjacent to the light source 11. The incident white light is totally reflected by the top surface and a bottom surface of the light guide plate 12 and then propagates to a second side surface of the light guide plate 12 opposite to the first side surface. Since the micro pattern 13 is formed on the top surface of the light guide plate 12 as illustrated in FIG. 1, some of the light incident on the top surface of the light guide plate 12 is diffracted by the micro patterns 13 to be emitted from the top surface of the light guide plate 12. The micro pattern 13 may be sinusoidally shaped and having a predetermined period. The light emitted from the top surface of the light guide plate 12 is uniformly diffused by the diffuser sheet 15, and illuminates a non-emissive display such as a liquid crystal display (LCD).
Since the light is composed of components with different refractive indices and transmittance, chromatic dispersion occurs when the light is emitted from the top surface of the light guide plate 12 through the micro pattern 13.
FIGS. 2A through 2C are diagrams for explaining chromatic dispersion. Luminance distributions of red (R) light, green (G) light, and blue (B) light among light emitted from the light guide plate 12 were measured. The micro patterns 13 had a period of 420 nm and a thickness of 250 nm. It was assumed that the red (R) light, the green (G) light, and the blue (B) light had wavelengths of 620 nm, 540 nm, and 450 nm, respectively. The width of the white light beam was 4 μm, and the white light was incident on the top surface 12 at an angle of 60°.
FIG. 2A illustrates luminance distribution of the red (R) light. The red (R) light was mostly distributed close to a central point. FIG. 2B illustrates luminance distribution of the green (G) light. The green (G) light was also distributed close to the central point but the brightest portion was located at a short distance from the central point. FIG. 2C illustrates luminance distribution of the blue (B) light. The brightest portion was located at a long distance from the central point. Accordingly, luminance distributions of light are different depending on the colors of light.
FIG. 3 is a graph illustrating a relationship between luminance distributions of the red (R) light, the green (G) light, and the blue (B) light and angles at which the light is emitted. Chromatic dispersion might be more clearly understood from the graph of FIG. 3. Here, the angles at which the light was emitted were measured from an imaginary reference line perpendicular to the top surface of the light guide plate 12. Referring to FIG. 3, the blue (B) light was distributed around an angle of about 22°, which is called a peak angle. That is, the blue (B) light was emitted from the light guide plate 12 at the peak angle of about 22°. The green (G) light was emitted from the light guide plate 12 at a peak angle of about 8°, and the red (R) light was emitted from the light guide plate 12 at a peak angle of about 4°.
Accordingly, since the blue (B) light, the green (G) light, and the red (R) light were emitted from the light guide plate 12 at different peak angles, the white light emitted from the point light source 11 was separated into colors by the light guide plate 12, thereby leading to chromatic dispersion. Even though the light subjected to the chromatic dispersion is uniformly diffused by the diffuser sheet 15, the light cannot be perfect white light. As a result, since the light incident on the display device is not white light but rather specific colors of light, the performance of the display device is degraded.